The IEEE 802.16e standard supports mobility, paging and fast call setup for a variety of terminals, including battery powered mobile stations. The mobile stations can include cellular telephones, personal digital assistants (PDAs), computers equipped with wireless transceivers (such as integrated transceivers or transceivers on PCMCIA cards), pagers, and the like.
FIG. 1 illustrates an exemplary orthogonal frequency division multiple access (OFDMA) frame with Time Division Duplexing (TDD) as the main physical layer (PHY) mode used for mobility within IEEE 802.16e. The horizontal axis represents time, while the vertical axis represents frequency tones or subcarriers. An OFDMA frame in a TDD system is divided into downlink and uplink subframes. Between the downlink and uplink subframes of a particular frame is a transmit transmission gap (TTG), which provides time for a base station to transition from a transmission mode to a receive mode and mobile stations to transition from a receive mode to a transmission mode. A receive transmission gap (RTG) is provided between an uplink subframe of one frame and the downlink subframe of a subsequent frame. The RTG provides base stations time to transition from a receive mode to a transmission mode and mobile stations to transition from a transmission mode to a receive mode.
In the frame of FIG. 1, the downlink subframe includes a preamble, frame control header (FCH), downlink MAP (DL-MAP), uplink MAP (UL-MAP) and downlink data channels. The preamble is used by mobile stations for cell acquisition and frame synchronization. The FCH is a broadcast channel located right after preamble, and includes information indicating the size of DL-MAP, repetition coding and forward error correction (FEC) coding used in DL-MAP, and other information related to the current frame. The DL-MAP, which follows immediately after FCH, includes a number of information elements (IEs), which define the usage of the downlink data channels and includes information such as frame synchronization, paging messages, downlink channel allocations and configuration change counts. The UL-MAP includes a number of information elements, which define the usage of the uplink data channels, and includes information such as uplink channel allocations, and uplink configuration change counts. The downlink data channels are used to transport data from a base station to other network nodes, e.g., fixed or mobile stations. The uplink subframe includes the uplink data channels, as well as ranging and uplink control channels.
The IEEE 802.16e standard defines active and non-active states for mobile stations. Idle and sleep modes are two non-active states defined in the IEEE 802.16 standard. While in idle or sleep modes, a mobile station will alternate between an unavailability interval, where the mobile station will not receive messages from the network, and an availability interval, where the mobile station will monitor and receive messages from the network.
Idle mode allows a mobile station, while traversing an air link environment populated by multiple base stations, to become periodically available for downlink broadcast traffic messaging without registration at a specific base station. While in the idle mode, a mobile station does not have to satisfy active state requirements, including handover. By restricting mobile station activity to scanning during availability intervals for downlink broadcast traffic messaging, idle mode allows the mobile station to conserve power and operational resources. Idle mode benefits the network and base stations by providing a simple method for alerting mobile stations to pending downlink traffic, and by eliminating air-interface and network handover traffic from an essentially inactive mobile station.
Mobile stations in sleep mode negotiate unavailability intervals with the network. During a mobile station's unavailability interval, the base station may buffer (or it may drop) medium access control (MAC) packets addressed to unicast connections toward the sleep mode mobile station. Additionally, the base station may choose to delay transmission of packets addressed to multicast connections until an availability interval, common for all mobile stations participating in the multicast connection. For each mobile station involved in a multicast connection, the base station maintains one or several contexts, each related to a certain Power Saving Class. A Power Saving Class is a group of connection identifications (CIDs) used by mobile stations which have common demand properties.
During availability intervals, a sleep mode mobile station is expected to receive all downlink transmissions, as in the active state. In addition, the mobile station examines the downlink channel descriptor (DCD) and uplink channel descriptor (UCD) change counts to determine whether there will be a change in the downlink or uplink portion of the frame, and the frame number of the DL-MAP PHY Synchronization Field, to verify synchronization with the base station. Sleep mode minimizes mobile station power usage, and decreases usage of base station air-interface resources.
As discussed above, during availability intervals, mobile stations in inactive states must parse through the entire set of messages in the downlink and uplink MAP to determine if there are any paging messages (for idle mode mobile stations), traffic channel allocations (for sleep mode mobile stations), or whether there are any configuration changes (i.e., DCD or UCD changes).
Parsing all of the messages in the DL- and UL-MAP requires considerable amount of processing at the physical layer (PHY), such as fast Fourier transform (FFT) operation, demodulation, de-interleaving, decoding, as well as MAC layer processing. Additionally, the size of the messages can be quite large. For example, the IEEE 802.16e standard allows transmission of 104 bits of broadcast information, several extended information elements, and for many active users several CID allocations. These CID allocations can be basic CIDs, primary or secondary CIDs, and traffic CIDs. Accordingly, a mobile station may parse through the entire set of messages in the DL- and UL-MAP and determine that there are no messages for the mobile station in these fields. The process of parsing the entire downlink and uplink MAP can consume considerable amount of power. Because mobile stations have a limited power supply, such as a rechargeable battery, parsing the entire set of messages will decrease the operating time of the mobile station, and in turn, require the battery to be recharged more often. Accordingly, it would be desirable to minimize the amount of processes for idle and sleep mode mobile stations.